This invention relates to an ink-jet printing method and apparatus for performing printing by jetting ink onto a recording medium from a print head.
A recording apparatus such as a printer, copier or facsimile machine is adapted to form dots on a recording medium such as paper or thin plastic sheets by individual recording elements (nozzles, heating elements or wires, etc.), thereby printing an image comprising the dots. Such a recording apparatus can be classified, depending on the recording technique used, as an apparatus of ink-jet type, wire dot type, thermal type or laser beam type, etc. Among these, the apparatus of the ink-jet type (referred to as an ink-jet printer) is so adapted as to jet ink (the recording fluid) from the nozzles of a print head and cause the jetted ink to adhere to a recording medium to thereby form an image on the medium.
A number of studies have been made for the purpose of improving the tonality of a color graphics output when printing a color image using such an ink-jet printer. For example, an improvement proposed and put into practice in recent years involves either raising print performance by making printing resolution higher than that of an ordinary color printing mode or raising the resolution of the printing apparatus, sending multilevel image data to the printing apparatus as print data and providing a multilevel printout using dots of a size different from the ordinary dot size. (The dots of the different size are referred to as "subpixels".)
An example of a method using subpixels is one which prints an image using mixed dots, namely dots of large and small size. Such a printing method makes it possible to improve tone reproducibility in image formation. However, though this method can be implemented with ease if the number of nozzles possessed by the ink-jet head is one per color, control becomes very complex if use is made of a head having a plurality of nozzles for each color.
In order for each nozzle to jet ink, the nozzle ordinarily is driven at a frequency above several kilohertz. Though direct control by a CPU is possible if the number of nozzles possessed by a head is small, it becomes increasingly necessary in view of processing speed to make joint use of hardware circuitry such as a gate array as the number of nozzles is increased. Further, if the amount of ink jetted from the nozzles is to be modulated to form large and small dots, this is carried out by changing the width of the driving pulses for ink discharge or by changing over the timing at which the driving elements within the nozzle are driven to discharge the ink. If drive timing is changed over, it is required that the head be internally provided with two drive-data registers per nozzle, one register being for large dots and one for small dots. If the number of nozzles is increased, the number of registers also increases by a whole-number multiple. The end result is print head circuitry of large scale and an attendant increase in the cost of the print head.
The method of changing the driving pulse width requires the provision of individual signal lines in order to control the nozzles individually. Consequently, the single signal line that usually suffices becomes several hundred signal lines (which is equivalent to the number of nozzles). This makes necessary an equivalent number of contacts between the head and its cable, an equivalent number of lines in the flexible cable leading to the print head and an equivalent number of driver transistors for the recording elements. This in turn leads to much higher cost.
If one foregoes printing by mixing large and small dots by way of single scan of a serial print head, then printing is carried out by causing the print head to make a plurality of scans (multiple passes) and combining scans which form large dots and scans which form small dots. Such method makes it possible to print by mixing large and small dots in an image through a simple arrangement. However, since this method always requires plural scans of the print head, a longer period of time is needed for printing.
Further, in a situation where pixels are thus formed using large and small dots, it is desirable to form the pixels by causing the large and small dots to overlap. However, a problem which arises is that the large and small dots are formed at positions offset from one another. This causes the image to take on a grainy appearance owing to small dots formed in spaced-apart relation and results in the appearance of white stripes.